The present invention relates to programming phase change memory cells.
Phase change memories use a class of materials that switch between two phases having distinct electrical characteristics, associated with two different crystallographic structures, and precisely an amorphous, disorderly phase and a crystalline or polycrystalline, orderly phase. The two phases are hence associated with resistivities of different values.
Currently, the alloys of elements of group VI of the periodic table, such as Te or Se, referred to as chalcogenides or chalcogenic materials, can be used advantageously in phase change memory cells. The currently most promising chalcogenide is formed from an alloy of Ge, Sb and Te (Ge2Sb2Te5).
The resistivity of phase change materials may vary by several orders of magnitude upon switching between the fully set (crystalline) state to the fully reset (amorphous) state. However, the resistivity of the amorphous chalcogenic material is not stable and continuously increases after phase transition. Thus, a quite rapid resistivity drift may take place, especially when large extensions of chalcogenic material are brought to the amorphous state.
The resistivity drift does not normally cause major problems in conventional two-level phase change memory cells because of the gap between the set state and the reset state. Multilevel programming is not compatible with the resistivity drift because of the smaller gap between the intermediate programming levels. Thus, a sense amplifier may fail to distinguish adjacent levels in a relatively short time after each phase transition. Moreover, large amorphous regions are created that unpredictably affect the resistivity level. Thus, repeating identical programming cycles on the same phase memory change cell may lead to different resistivity levels.
Thus there is a need for other ways to implement multilevel phase change memories.